I took the idea from this tutorial by Jules1983 . You can also find an interesting setup in melvynyeo's blog here: melvyn-yeo.blogspot.com.es/p/a…, just scroll down a bit.
All you have to do is to cut any kind of package like the one in the images below, the hole at the bottom is to insert the flash, so it must bit the exact size of it not to loose any light from there. The trick is that this kind of packages interior are made of aluminum, what transmit light very well.
This is the basic setup of the flash modifier
After that it will be a good idea to find some kind of "translucent foam" or some kind of cloth, wich would let pass most of the light but wich won't create hard shadows.
This is the modifier mounted on the camera with and without the "diffuser"
I hope this to be helpful to someone who wants to try macro photography, and please forgive the crappy pictures.